The aim of this project is to clone and characterize the genes responsible for the Bare Lymphocyte Syndrome (BLS), a Severe combined Immunodeficiency (SCID). These patients die in the first few years of life unless they obtain a successful bone marrow transplant. The genes for class II major histocompatibility antigens (MHC) are present but not expressed in BLS cells. There are three, as yet unidentified, genetic defects any one of which can lead to the BLS. Mouse, DNA transfected into BLS cells c,an restore human class II MHC expression. In this project, one member of each group of class II negative B cell lines will be cotransfected with labelled mouse genomic DNA and class II promoter DNA linked to a selectable marker. These cells will then be grown in selective media and class II positive clones will be isolated by cell sorting. The labelled mouse DNA will be amplified by polymerase chain reaction using the label as a primer This DNA will be characterized and transfected into class II negative cells to verify its ability to allow class II expression Identification of these genes will increase our understanding of the Bare Lymphocyte Syndrome (possibly leading to gene replacement therapy) and could lead to the development of new drugs able to modify the specificity or intensity of the immune response to be used in the treatment or prevention of infectious cancerous or autoimmune diseases.